The probability of correct forecasting is something that has been
rattling around in my head for a while.

If on Day 1 of a 5 day forecast I'm told Rain is on the way only for
this forecast to change between rain/dry (for example) over the next
three days and then its actually dry on day 5, the forecast on day 1
was utterly worthless.

What probability of a likely event happening triggers it becoming a
'forecast'. Surely it needs to be 75% likely as that would mean it is
twice as likely to happen, as not.

Also why is that probabilty of a forecast never mentioned on tv/radio/
media forecasts. Is it because Jo Public has dumbed down so much over
the last few years that he has no concept what Porbability is?